A variety of thermoplastic materials and compositions have been developed and used in the roadway striping industry. In order to apply such thermoplastic materials and compositions, they have to be melted and mixed. Melting, which involves both initial melting from solid stock or feed materials and maintaining the materials/compositions in a molten state for application onto roadways and other pavements, is typically conducted in melter kettles which can be heated by electrical means, or combustionable fuels.
Pavement striping apparatus which are used to apply thermoplastic lane markers and other roadway and pavement informational markers include walk behind systems, systems that are pulled on trailers, and systems that are built on vehicle chassis. In the case of heavy duty commercial equipment, pavement striping apparatus or systems are built on the chassis of large trucks and can include one or more large capacity melter kettles. Such melter kettles can be quite tall and provided with various structures such as platform steps, ladder rungs, bridges, etc. by which operators or tenders can access and charge material into the tops of the melter kettles.
Traditionally thermoplastic melter kettles have been loaded manually. A person referred to herein as a tender lifts or otherwise transfers plastic bags of granular or chunk thermoplastic material to the melter kettle and introduces the material into melting kettle by releasing the material such that it falls through an opening on the top of the melter kettle. The material free falls into the melter kettle in its packaged state or solid state. Mechanical agitators in the melter kettle mix the material. The thermoplastic material can be a bagged granular material or solid block material of various sizes. Other material added can be, but is not limited to, glass beads, resins or oils. The opening at the top of the melter kettle into which the thermoplastic and other materials are feed into the melter kettle can be a hinged lid or any number of various design chutes with baffle doors.
The hinged lid offers the person or tender feeding material into the kettle no protection against splash back of molten material or blow back of flames from super heated material. When the melter kettle lid is opened while the thermoplastic is above its flashpoint oxygen is sucked into the melter kettle and ignites with an explosive force. Serious injury can be caused to anyone in the influence of the blast. Various designed baffle chutes offer some protection against splash back of material; however, blow back of flames from super heated material remains a constant danger. These hazards are serious deficiencies that need to be corrected.
There are additional problems that can occur when material is fed into melter kettles using the traditional methods. If the material is added in solid form it will melt slowly. This is because the surface area of the solid material in contact with the heated walls and floor of the melter as well as molten material in the kettle is small relative to its mass. The agitators in the kettles do not have the ability to break the solid material into smaller pieces. This material can only melt from its outer surface. This slow liquification to optimum application temperature increases both production time and job cost.
Thermoplastic material that is packaged in meltable bag form often is in a semisolid state. As stored material ages the effects of temperature and the weight of the material due to stacking causes the material to clump together. It is difficult for the agitators in melter kettles to break this material apart. Melting of this material is slower than it would be if the material were uniformly granular.
Thermoplastic bagged granular material that is in the ideal condition breaks apart readily but not immediately under agitation in the kettle. If this material were added as granules to the kettle then an optimum melt time can be achieved.
Thermoplastic material that reaches and exceeds its flashpoint must be cooled rapidly. This is for two reasons. First, superheated material can ignite with serious consequences when exposed to air. Second, the thermoplastic can break down from the effects of this heat in a short time and become waste material.
Overheated or super heated material can be cooled by the addition of unheated material to the kettle. Material added to the kettle using traditional methods as described herein has a low efficiency of cooling.
The present invention allows for the safe addition of uniformly granular cooling material into the melter kettle and provides a greater degree of cooling efficiency than is possible with current state of the art systems.